Understanding the surface segregation of solute atoms in Sn-Bi–based solder from first principles

Abstract

Low-temperature Sn-Bi solder has wide application in the field of electronic packaging due to its low melting point and good wettability. The formation of Bi-rich phase and intermetallic compound is the major concern for the reliability of Sn-Bi solder joints. We employed first-principles calculations to understand the segregation of Bi and the third elements to the surface of Sn. The effects of alloying elements on inhibiting the Bi surface segregation were described. Our calculations show that the Bi surface segregation could be effectively alleviated by the addition of Ag, Ga, Ni, and In, along with the reduction of further possible formation of intermetallic compounds in the Sn-Bi–based solders. The results could be interpreted by the enhanced bond orders between Bi and its neighboring Sn, alloying elements.